Synthetic approaches and pharmacological activity of 1,3,4-oxadiazoles: a review of the literature from 2000-2012.

This review provides readers with an overview of the main synthetic methodologies for 1,3,4-oxadiazole derivatives, and of their broad spectrum of pharmacological activities as reported over the past twelve years.

1. Introduction

1,3,4-Oxadiazole (1, Figure 1) is a heterocyclic compound containing an oxygen atom and two nitrogen atoms in a five-membered ring. It is derived from furan by substitution of two methylene groups (=CH) with two pyridine type nitrogens (-N=) [1,2]. There are three known isomers: 1,2,4-oxadiazole (2), 1,2,3-oxadiazole (3) and 1,2,5-oxadiazole (4) (Figure 1). However, 1,3,4-oxadiazole and 1,2,4-oxadiazole are better known, and more widely studied by researchers because of their many important chemical and biological properties.

Figure 1

Isomers of oxadiazole.

Among heterocyclic compounds, 1,3,4-oxadiazole has become an important construction motif for the development of new drugs. Compounds containing 1,3,4-oxadiazole cores have a broad biological activity spectrum including antibacterial, antifungal, analgesic, anti-inflammatory, antiviral, anticancer, antihypertensive, anticonvulsant, and anti-diabetic properties. They have also attracted interest in medicinal chemistry as surrogates (bioisosteres) for carboxylic acids, esters and carboxamides [2]. The ability of 1,3,4-oxadiazole heterocyclic compounds to undergo various chemical reactions has made them important for molecule planning because of their privileged structure, which has enormous biological potential. Two examples of compounds containing the 1,3,4-oxadiazole unit currently used in clinical medicine are: Raltegravir® (5), an antiretroviral drug [3] and Zibotentan® (6) an anticancer agent [4] (Figure 2).

Figure 2

Structures of raltegravir and zibotentan, drugs that are in late stage clinical development.

The synthesis of novel 1,3,4-oxadiazole derivatives, and investigation of their chemical properties and biological behavior has accelerated in the last two decades. In recent years the number of scientific studies with these compounds has increased considerably. Considering the period from 2002 to 2012, the Scifinder Scholar database records 2,577 references to 1,3,4-oxadiazole, demonstrating its relevance for heterocyclic chemistry. Figure 3 shows the number of publications over the past twelve years involving 1,3,4-oxadiazole. The graph is of course not linear, there is a decrease from 2002 (169 articles) to 2003 (146 articles), and then a gradual increase from 2003 to 2006 (219 articles), again a small decline from 2006 to 2007 (214 articles), and an increase from 2007 to 2011 (319 articles) [5].

Figure 3

Number of publications in the last twelve years involving 1,3,4-oxadiazole.

Taking into account the importance of these compounds to both heterocyclic and medicinal chemistry, we have decided to present the main synthesis approaches used for obtaining the heterocyclic nucleus, as well as the broad spectrum of pharmacological activities reported in the literature over the past twelve years.

2. Methods of Synthesis for 2,5-Disubstituted-1,3,4-oxadiazoles

2.1. Methods of Synthesis for 5-Substituted-2-amino-1,3,4-oxadiazoles

A few of the methods reported in the literature for the preparation of 5-substituted-2-amino-1,3,4-oxadiazoles 7 are outlined in Scheme 1. Methods a and b use the acylhydrazide intermediate 8, readily prepared from the corresponding ester and hydrazine hydrate, which can react with cyanogen bromide (14), or di(benzotriazol-1-yl)methanimine (23). Dehydration of acylsemicarbazide 9 has also been used extensively, although more stringent conditions are necessary (method c). Acylthiosemicarbazide intermediates 11 and 12 have been used in different routes to obtain the desired heterocycles through oxidative cyclization reactions with iodine at elevated temperatures, or by carbodiimide derivatives (methods e and f). Semicarbazones 10, being versatile intermediates, can easily be cyclized to the corresponding 1,3,4-oxadiazoles (method d).

Scheme 1

Retrosynthetic analysis of 5-substituted-2-amino-1,3,4-oxadiazole.

Using approach (a) of Scheme 1, Patel and Patel [6] synthesized 5-aryl-2-amino-1,3,4-oxadiazole compounds 15 in yields of 62 to 70%. These compounds were used as intermediates for the synthesis of new quinazolinone derivatives (Entry a, Scheme 2). Kerimov and co-workers [7] synthesized a new series of 2-amino-1,3,4-oxadiazoles 17 carrying a benzimidazole moiety in 33%–60% yield from the reaction between 2-(2-(4-substituted-phenyl)-1H-benzo[d]imidazol-1-yl)acetohydrazide (16) and cyanogen bromide, (Entry b, Scheme 2).

Scheme 2

5-Aryl-2-amino-1,3,4-oxadiazole obtained from acylhydrazides and cyanogen bromide.

Katritzky and co-workers [8] have prepared 5-aryl-2-amino-1,3,4-oxadiazole compounds 18 (Scheme 3) in excellent yields from the reaction between di(benzotriazol-1-yl)methanimine (13) and arylhydrazides 8 using approach (b) of Scheme 1.

Scheme 3

5-Aryl-2-amino-1,3,4-oxadiazole obtained from acylhydrazides and di(benzotriazol-1-yl)methanimine.

The oxidative cyclization of semicarbazones 19 (Scheme 4) with bromine in acetic acid is one of the approaches frequently used for the preparation of 5-substituted-1,3,4-oxadiazol-2-amines 20 (Entry a, Scheme 4) [9,10]. Electro-cyclization of semicarbazones 21 to their corresponding 5-aryl-2-amino-1,3,4-oxadiazoles 22 has emerged as an alternative method (Entry b, Scheme 4) [11,12].

Scheme 4

5-Substituted-1,3,4-oxadiazol-2-amines from of cyclization reaction of semicarbazones.

Another interesting approach to the synthesis of 5-substituted-2-amino-1,3,4-oxadiazoles is the cyclization reaction of acylthiosemicarbazides using iodine as the oxidizing agent. El-Sayed and co-workers [13] reported the synthesis of 5-((naphthalen-2-yloxy)methyl)-N-phenyl-1,3,4-oxadiazol-2-amine (24) in 62% yield, by heating compound 23 in ethanol in the presence of sodium hydroxide and iodine in potassium iodide (Scheme 5).

Scheme 5

Synthesis of 1,3,4-oxadiazol-2-amines from of cyclization reaction of acylthiosemicarbazides with iodine.

Rivera and co-workers [14] reported that 1,3-dibromo-5,5-dimethylhydantoin is an effective oxidizing agent for cyclization reactions of acylthiosemicarbazide. Compounds 25 were cyclized to 5-aryl-2-amino-1,3,4-oxadiazoles 26 in excellent yield (Scheme 6). The main advantage of this method is that the reagents used are commercially cheap and safe to work with. Further, it is applicable to large scale synthesis where other oxidizing agents cannot be used.

Scheme 6

Synthesis of 5-aryl-2-amino-1,3,4-oxadiazole from acylthiosemicarbazide and 1,3-dibromo-5,5-dimethylhydantoin.

In general the 5-aryl(alkyl)-2-amino-1,3,4-oxadiazoles can be prepared by dehydration of derivatives of semicarbazides or thiosemicarbazides using POCl3 as dehydrating agent. Alternative reagents that activate the carbonyl group have also been used. Accordingly, Dolman and co-workers [15] reported a new method of synthesis for 5-aryl(alkyl)-2-amino-1,3,4-oxadiazoles 28 from acylsemicarbazides 27 (X=O) and acylthiosemicarbazides 27 (X=S) mediated by tosyl chloride. Yields of 97%–99% were obtained when using thiosemicarbazide derivatives which are more reactive than the corresponding semicarbazide derivatives (Scheme 7).

Scheme 7

Synthesis of 5-aryl-2-amino-1,3,4-oxadiazole from acylthiosemicarbazide and tosyl chloride.

2.2. Methods of Synthesis for 5-Substituted-1,3,4-oxadiazole-2-thiols

The main synthesis route for 5-substituted-1,3,4-oxadiazole-2-thiol(thione)s 29 involves an initial reaction between an acylhydrazide 8 and carbon disulfide in an basic alcohol solution, followed by acidification of the reaction mixture (Scheme 8). A large number of 1,3,4-oxadiazole derivatives prepared by this route have been reported in recent years. The existence of thiol-thione tautomerism is known for the compounds 29, and one of the forms usually predominates [16].

Scheme 8

Synthesis of 5-substituted-1,3,4-oxadiazole-2-thiols.

The compounds (30) [17], (31) [18], (32) [19], (33) [20], (34) [21], (35) [22], (36) [23], (37) [24], (38) [16] and (39) [25] (Figure 4) are just some of many compounds of this class prepared using the synthetic route shown in Scheme 8.

Figure 4

5-Aryl-1,3,4-oxadiazole-2-thiols obtained by reaction of acylhydrazide with carbon disulfide.

2.3. Methods of Synthesis for 2,5-Diaryl(alkyl)-1,3,4-oxadiazole

Several methods of synthesis have been reported in the literature for the preparation of both symmetrical and asymmetrical 2,5-diaryl(alkyl)-1,3,4-oxadiazoles 40 (Scheme 9). One of the most popular methods involves the cyclodehydration of 1,2-diacylhydrazines 41 using phosphorous oxychloride (POCl3) as a dehydrating agent, path (c) of Scheme 9. Other dehydrating agents commonly used are sulfuric acid, phosphoric acid, trifluoroacetic acid, phosphorus pentachloride, phosphorus pentoxide, thionyl chloride, and milder reagents such as carbodiimide derivatives, TsCl/pyridine, trimethylsilyl chloride, Ph3O/Tf2O, PPh3/CX4 (X = Cl, Br, I), and Burgess reagent. Other important routes for obtaining either symmetrical (R = R1) or asymmetric (R ≠ R1), 2,5-diaryl(alkyl)-1,3,4-oxadiazoles 40 are reactions of acylhydrazides 8 with aromatic carboxylic acids 42 (a), oxidative cyclization of acylhydrazones 45 (b), and the reaction of tetrazoles 43 with acid chlorides 44 in the presence of pyridine, path (d) (Scheme 9).

Scheme 9

Retrosynthetic analysis of 2,5-diaryl(alkyl)-1,3,4-oxadiazoles.

The asymmetrical 5-(2,4-dichloro-5-flurophenyl)-2-(aryl)-1,3,4-oxadiazole compounds 47 were prepared in two steps (Scheme 10) by refluxing the corresponding diacylhydrazines 46 with phosphorus oxychloride as reported by Zheng and co-workers [26]. If not intending to isolate the intermediate diacylhydrazine, generally the one-pot reaction of a carboxylic acid with acylhydrazide and POCl3 as dehydrating agent is used (see approach a, Scheme 9). Similarly, Amir and Kumar [27] reported the synthesis of novel 2,5-disubstituted-1,3,4-oxadiazole derivatives 49 beginning with the anti-inflammatory drug ibuprofen as a starting material (Scheme 11). Phosphorus oxychloride (POCl3) was used as a dehydrating agent in the reaction of acylhydrazide 48 with substituted aromatic carboxylic acids.

Scheme 10

Synthesis of asymmetrical 2-(2,4-dichloro-5-fluorophenyl)-5-aryl-1,3,4-oxadiazoles.

Scheme 11

Synthesis of 2,5-dissubstituted-1,3,4-oxadiazole derivatives of ibuprofen.

Another dehydrating agent normally used for the dehydration of diacylhydrazines is thionyl chloride; Scheme 12 outlines some examples.

Scheme 12

Cyclization of diacylhydrazine with thionyl chloride. 50 [28], 51 [29] and 52 [30].

The use of POCl3 requires care because it is very toxic and corrosive. Less dangerous reagents (that are easier to work with) than POCl3 have arisen in recent years. For example, Bostrom and co-workers [2] synthesized 2,5-disubstituted-1,3,4-oxadiazole compounds 54 by cyclodehydration of diacylhydrazine 53 using triphenylphosphine oxide (3 equivalents), and triflic anhydride (1.5 equivalents), obtaining from 26 to 96% yields (Scheme 13).

Scheme 13

Cyclodehydration of diacylhydrazine using triphenylphosphine oxide and triflic anhydride.

Nagendra and co-workers [31] reported the synthesis of novel orthogonally protected 1,3,4-oxadiazole 56, tethered dipeptide mimetics, by cyclodehydration of diacylhydrazine 55 using 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) as a dehydration agent, obtaining a 70%–92% yields (Scheme 14).

Scheme 14

Cyclodehydration reaction of diacylhydrazines using EDC.

Li and co-workers [32] reported that silica-supported dichlorophosphate is an efficient cyclodehydration agent for the synthesis of 2,5-disubstituted 1,3,4-oxadiazoles 58 from 1,2-diacylhydrazines in solvent-free medium under microwave irradiation. This protocol was suitable for the synthesis of alkyl, aryl, and heterocyclic substituted symmetrical and unsymmetrical 1,3,4-oxadiazoles, and has the specific advantages of no corrosion or environmental pollution, an accelerated rate, high yield and a simple work-up procedure (Scheme 15).

Scheme 15

Synthesis of 2,5-disubstituted 1,3,4-oxadiazoles from 1,2-diacylhydrazines and silica-supported dichlorophosphate.

Sharma and co-workers [33] developed a simple generalized method for the synthesis of 1,3,4-oxadiazoles 59 from diacylhydrazines using inexpensive ZrCl4 as a catalyst. Advantages over the existing methods include higher yields, shorter reaction times, and a simple experimental procedure (Scheme 16).

Scheme 16

Synthesis of 2,5-disubstituted 1,3,4-oxadiazoles from 1,2-diacylhydrazines and zirconium(IV) chloride.

Yang and Shi [34] reported the effect of halogens in a Robinson-Gabriel type reaction of cyclopropane-carboxylic acid N'-substituted-hydrazides with PPh3/CX4 (X = Cl, Br, I) as dehydration agents resulting in the formation of 1,3,4-oxadiazoles 60 (Scheme 17).

Scheme 17

Effect of halogens in the formation of 1,3,4-oxadiazoles.

Pouliot and co-workers [35] reported the use of diethylaminodifluorosulfinium tetrafluoroborate ([Et2NSF2]BF4), XtalFluor-E, as a new cyclodehydration agent for the preparation of 1,3,4-oxadiazoles 61 from 1,2-diacylhydrazines (Scheme 18).

Scheme 18

Preparation of 1,3,4-oxadiazoles from 1,2-diacylhydrazines using XtalFluor-E.

Li and Dickson [36] developed a convenient one-pot protocol for the synthesis of 1,3,4-oxadiazoles 62 from carboxylic acids and hydrazides using HATU as coupling agent and Burgess reagent as dehydrating agent (Scheme 19).

Scheme 19

Synthesis of 1,3,4-oxadiazoles from carboxylic acids and hydrazides using HATU and Burgess reagent.

Another method for one pot synthesis of 2,5-disubstituted-1,3,4-oxadiazoles 63 from benzohydrazide and carboxylic acid was reported by Rajapakse [37] using the coupling agent 1,1'-carbonyldiimidazole (CDI) and triphenylphosphyne as dehydrating agent (Scheme 20).

Scheme 20

Synthesis of 2,5-dissubstituted-1,3,4-oxadiazoles using CDI and triphenylphosphyne.

In 2006, Kangani and co-workers [38] described a one-pot direct synthesis of 1,3,4-oxadiazoles 64 in excellent yields from carboxylic acids (1 equiv) and benzohydrazide (2.2 equiv) using Deoxo-Fluor reagent (Scheme 21).

Scheme 21

Synthesis of 1,3,4-oxadiazoles using Deoxo-Fluor.

A convenient, one pot procedure was also reported for the synthesis of a variety of 2,5-disubstituted-1,3,4-oxadiazoles 65 by condensing monoarylhydrazides with acid chlorides in HMPA solvent under microwave heating. The yields were good to excellent, the process was rapid, and needed no additional acid catalyst or dehydrating reagent (Scheme 22) [39].

Scheme 22

Synthesis of 2,5-disubstituted-1,3,4-oxadiazoles using microwave heating.

In approach b (see Scheme 10), N-acylhydrazones generally suffer oxidative cyclization under the action of oxidizing agents such as Br2, HgO, KMnO4 and acetic anhydride. Other milder oxidizing agents have emerged in recent years such as ammonium cerium nitrate, Cu(OTf)2, chloramine-T, trichloroisocyanuric acid and the hypervalent iodines. For example, the reaction of N-acylhydrazones 66 treated with acetic anhydride under reflux conditions gives compounds 67 in good yields (Scheme 23) [40].

Scheme 23

Synthesis of 1,3,4-oxadiazolines from N-acylhydrazones using acetic anhydride.

In 2006, Dabiri and co-workers [41] reported a new procedure for the synthesis of disubstituted oxadiazoles 68 through a one-pot reaction of benzohydrazide, and para substituted aromatic aldehydes in the presence of an cerium ammonium nitrate (CAN) and dichloromethane solvent (Scheme 24).

Scheme 24

Synthesis of 2,5-diaryl-1,3,4-oxadiazoles from benzohydrazide and aromatic aldehydes.

Guin and co-workers [42] reported a direct route to both symmetrical and unsymmetrical 2,5-disubstituted-1,3,4-oxadiazoles 70 by means of an imine C-H functionalization of N-arylidenearoylhydrazide 69 using Cu(OTf)2 as catalyst (Scheme 25).

Scheme 25

Synthesis of 1,3,4-oxadiazoles from N-arylidenearoylhydrazides and Cu(OTf)2.

Li and He [43] synthetized the compound 2-(anthracen-9-yl)-5-(p-tolyl)-1,3,4-oxadiazole (72) in 75.4% yield from oxidative cyclization of N-acylhydrazone 71 using chloramine-T, (Entry a, Scheme 26). Gaonkar and co-workers [44] also reported the synthesis of 1,3,4-disubstituted oxadiazoles 74 from the oxidative cyclization of N-acylhydrazones 73 with chloramine-T under microwave irradiation, (Entry b, Scheme 26).

Scheme 26

Oxidative cyclization of N-acylhydrazones using chloramine-T.

Pore and co-workers [45] developed an efficient method for one-pot synthesis of unsymmetrical 2,5-disubstituted 1,3,4-oxadiazoles 75 using trichloroisocyanuric acid (TCCA) at ambient temperatures. The main advantages of this method are the mild nature of the synthesis, and the short reaction time (Scheme 27).

Scheme 27

Synthesis of 1,3,4-oxadiazoles using trichloroisocyanuric acid (TCCA).

Pardeshi and co-workers [46], using a mixture of N-chlorosuccinimide and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) oxidatively cyclized structurally diverse acylhydrazones 76, thereby providing an efficient and convenient method for the synthesis of various 2,5-disubstituted 1,3,4-oxadiazoles 77. The salient features of this method are the mild reaction conditions, short reaction time, excellent yields, and a simple workup procedure (Scheme 28).

Scheme 28

Oxidative cyclization of acylhydrazones using N-chlorosuccinimide and 1,8-diazabicyclo-[5.4.0]undec-7-ene (DBU).

Dobrotã and co-workers [47] reported the synthesis of 2,5-disubstituted-1,3,4-oxadiazoles 79, conveniently prepared by oxidative cyclization of N-acylhydrazones 78 through use of an excess of Dess-Martin periodinane under mild conditions (Scheme 29).

Scheme 29

Oxidative cyclization of N-acylhydrazones using Dess-Martin periodinane.

Polshettiwar and Varma [48] reported a novel one-pot solvent-free synthesis of 1,3,4-oxadiazoles 82 by condensation of benzohydrazide 80 and triethylorthoalkanates 81 under microwave irradiation, and efficiently catalyzed by Nafion®NR50 (solid supported), and phosphorus pentasulfide in alumina (P4S10/Al2O3) with excellent yields (Scheme 30).

Scheme 30

Nafion catalized 1,3,4-oxadiazole synthesis.

Kudelko and Zieliǹski [49] developed an easy and efficient method to synthesize 5-substituted-2-styryl-1,3,4-oxadiazoles 85 from cinnamic acid hydrazide 83 and commercially available triethyl orthoesters 84. The method provides the desired products rapidly and in high yields making it a useful addition to the existing synthetic procedures (Scheme 31).

Scheme 31

Reaction of cinnamic acid hydrazide with triethyl orthoesters.

Cui and co-workers [50] reported the synthesis of various α-keto-1,3,4-oxadiazole derivatives through a sequential intermolecular (dehydrochlorination/intramolecular) aza-Wittig reaction of carboxylic acids and imidoyl chloride intermediates, which were generated through isocyanide-Nef reactions of acyl chlorides and (N-isocyanimine)triphenylphosphorane in CH2Cl2 at room temperature (Scheme 32).

Scheme 32

Synthesis α-keto-1,3,4-oxadiazole.

Ramazani and Rezaei [51] developed a novel and efficient method for the synthesis of 2,5-disubstituted-1,3,4-oxadiazoles 88 in high yield by a one-pot condensation procedure in CH2Cl2 at room temperature involving four components: (N-isocyanimino)triphenylphosphorane (87), a secondary amine, a carboxylic acid, and an aromatic aldehyde (Scheme 33). For similar reactions with their respective mechanisms, see references [52,53,54,55,56,57].

Scheme 33

Synthesis of dissubstituted 1,3,4-oxadiazoles from four components in a one-pot procedure.

Although less popular than the methods mentioned above, the Huisgen reaction (reaction of 5-aryl/acyltetrazoles with acid chlorides or acid anhydrides) is widely used for the synthesis of various 2,5-disubstituted-1,3,4-oxadiazoles. Some interesting examples are outlined below.

Tetrazole 89 with chloroacetyl chloride (90) give disubstituted oxadiazole 91(Entry a, Scheme 34) [58]. Reflux of 4-methoxyphenyltetrazole (92) with 4-tert-butylbenzoyl chloride for 2 h affords 2-(4-tert-butylphenyl)-5-(4-methoxyphenyl)-1,3,4-oxadiazole (93) in 96% yield (Entry b, Scheme 34) [59]. Similarly, compounds 95 (Entry c) [60] and 98 (Entry d) [61] are obtained in excellent yields when treating the intermediates 94 and 97 with acid chlorides (Scheme 34).

Scheme 34

Synthesis of 1,3,4-oxadiazoles by the Huisgein reaction.

The Huisgein reaction also proceeds well with acid anhydrides in place of acid chlorides, which was demonstrated by Efimova and co-workers [62], by synthesizing 1,3,4-oxadiazole compounds 100, and 101 by acylation of a series of 5-aryl(hetaryl)tetrazoles 99 with acetic and benzoic anhydrides under microwave irradiation conditions (Scheme 35) (see also Reichart and Kappe [63]).

Scheme 35

Acylation of tetrazoles with acetic and benzoic anhydrides.

3. Pharmacological Activity of 1,3,4-Oxadiazoles

3.1. Antimicrobial Activity

The recent emergence of drug resistance when treating infectious diseases has underlined the need for new, safer, and more efficient antimicrobial agents. Many researchers have reported excellent antimicrobial activity for compounds containing the 1,3,4-oxadiazole core.

Recently, Oliveira and co-workers [64] reported synthesis and antistaphylococcal activity of 1,3,4-oxadiazolines 102 against strains of Staphylococcus aureus, resistant to methicillin and amino glycosides (MARSA), and that encode efflux proteins (multidrug drugs resistant—MDR). The compounds 102 showed efficient antistaphylococcal activity at 4 to 32 μg/mL, making all the compounds 2–8 times more active than the standard drug chloramphenicol (Figure 5).

Figure 5

Disubstituted-1,3,4-oxadiazoles with antibacterial and antifungal activity.

A series of new derivatives of 5-(1-/2-naphthyloxymethyl)-1,3,4-oxadiazol-2(3H)-thione (R=SH), 5-(1-/2-naphthyloxymethyl)-1,3,4-oxadiazole-2-amino (R=NH2), and 5-(1-/2-naphthyloxymethyl)-1,3,4-oxadiazol-2(3H)-ones (R=OH) 103 [65] were synthesized and evaluated for their antimicrobial activity. All were active against S. aureus, E. coli, P. aeruginosa, C. albicans and C. parapsilosis at a minimum concentration of 64–256 mg/mL (Figure 5).

Patel and Patel [6] verified the antibacterial activity of a series of derivatives containing the 1,3,4-oxadiazole nucleus against Gram-positive (S. aureus MTCC 96 and S. pyogenes MTCC 442) and Gram-negative bacteria (E. coli MTCC 443 and P. aeruginosa MTCC 1688) using ampicillin as the drug standard. The compounds 4-[5-(2-chlorophenyl)-1,3,4-oxadiazol-2-yl]benzenamine (104), and 3-{[5-(2-chlorophenyl)-1,3,4-oxadiazol-2-yl]methyl}-2-{2-[2,6-dichlorophenyl)amino]benzyl}-6-iodoquinazolin-4(3H)-one (105) were respectively 2 and 5 times more potent than ampicillin (Figure 5).

2,5-Disubstituted oxadiazole compounds 106 (Figure 5) containing the acetyl group in position 3 of the oxadiazole ring were synthesized and evaluated against two strains of bacteria, S. aureus and P. aeruginosa, and against two species of fungi, C. albicans and A. flavus, by the disk diffusion method. Ampicillin and fluconazole were used as drug standards for the antibacterial and antifungal activity, respectively. In comparison, all of the compounds were equally potent to their ampicillin and fluconazole standards [66].

The antibacterial and antifungal activity of 2-(5-amino-1,3,4-oxadiazol-2-yl)-4-bromophenol (107), and 5-(3,5-dibromophenyl)-1,3,4-oxadiazol-2-amine (108) were investigated against two strains of Gram-positive bacteria; Streptococcus aureus, Bacillus subtilis, two strains of Gram-negative bacterial; Klebsiella pneumoniae and Escherichia coli, and two fungal species; Aspergillus Niger and C. Pannical. The tests showed activities which were approximately equal to the standard drugs of treatment streptomycin and griseofulvin, respectively, [67] (Figure 5).

Sangshetti and co-workers [68] investigated the antifungal activity of a number of disubstituted oxadiazoles 109 (Figure 5), each of which contained a triazole unit at position 5 of the oxadiazole ring. The species of fungi tested were Candida albicans, Fusarium oxysporum, Aspergillus flavus, Aspergillus Níger, and Cryptococcus neoformans. Miconazole and fluconazole were used as standards for the comparison. The compounds containing the methyl sulfone (R=SO2CH3) group attached to the nitrogen of the piperidine ring, and Cl or OH (R1) groups exhibited excellent pharmacological profiles (equal to miconazole) against some of the fungi.

Compounds 110 and 111 were respectively 2 and 4 fold more potent than furacin when evaluated against E. coli, and P. aeruginosa. Compounds 112 and 113 were twice as potent as fluconazole against C. albicans [69] (Figure 5). Other oxadiazole compounds with antibacterial activity are: 114 [70], 115 [71], 116 [72], 117 [13], 118 [73], 119 [24], 120 [74], 121 [75] and 122 [76] (Figure 5).

The compound 2-(2-naphthyloxymethyl)-5-phenoxymethyl-1,3,4-oxadiazole (123) exhibitsanti-mycobacterial activity at a minimum inhibitory concentration of 6.25 µg/mL (Figure 6) [77]. Anti-mycobacterial activity against Mycobacterium tuberculosis H37RV was also studied by Kumar and co-workers [40] for a series of di-substituted oxadiazoles 124 containing the thiazole unit. The derivative containing the Cl group exhibited excellent results at a minimum inhibitory concentration of 4 μg/mL (Figure 6).

Figure 6

1,3,4-Oxadiazoles with anti-mycobacterial activity.

Yoshida and co-workers [78] described the synthesis and optimization of anti-Helicobacter pylori activity for a new series of cephem derivatives. Compound 125 exhibited anti Helicobacter pylori (13001 and FP1757) activity at a minimum inhibitory concentration of 0.1 μg/mL. Bakal and Gattani [79] investigated anti-tubercular activity for a series of 2,5-disubstituted oxadiazoles against M. tuberculosis H337Rv. Compound 126 with a MIC50 = 0.04 ± 0.01 μM was comparable with Isoniazid. Compound 127 was 7.3-fold more active against Mycobacterium tuberculosis H37Rv, and 10.3-fold more active against INH resistant Mycobacterium tuberculosis than Isoniazid (Figure 6) [80].

3.2. Anticonvulsant Activity

New 3-[5-(4-substituted)-phenyl-1,3,4-oxadiazole-2-yl]-2-styrylquinazoline-4(3H)-one oxadiazoles 128 were synthesized and evaluated by Kashaw and co-workers [81] (Figure 7) for anticonvulsant activity. New 2-substituted-5-(2-benzylthiophenyl)-1,3,4-oxadiazole derivatives 129 were designed and synthesized as anticonvulsant agents. The authors found that introduction of an amino group at position 2 of the 1,3,4-oxadiazole ring, and a fluorine substitute at the para position of the benzylthio group improves anticonvulsant activity [82], (Figure 7).

Figure 7

1,3,4-Oxadiazoles with anticonvulsant activity.

Rajak and co-workers [83] synthesized and evaluated semicarbazones 130 containing the 1,3,4-oxadiazole units for anticonvulsant potential in a three model test (MES), (scPTZ) and (scSTY). Most of them showed activity in all three models, (Figure 7). We include other compounds with anticonvulsant activity: 131 [84], 132 [85], 133 [86], 134 [82], 135 [87], 136 [88], 137 [89] (Figure 7).

3.3. Anti-inflammatory Activity

A series of oxadiazole derivatives 138 of ibuprofen which contains the arylpiperazine unit at position 3 of the oxadiazole ring were investigated by Manjunatha and co-workers [20] for anti-inflammatory activity using paw edema induced by carrageenan as the method with sodium diclofenac as the reference. Compounds containing 4-Cl, 4-NO2, 4-F and 3-Cl groups were more active than sodium diclofenac, whereas compounds with 4-MeO and 2-EtO groups showed less activity (Figure 8).

Figure 8

1,3,4-Oxadiazoles with anti-inflammatory activity.

Compounds 139 were synthesized from the anti-inflammatory drug fenbufen and evaluated for anti-inflammatory activity by carrageenan induced paw edema; sodium diclofenac and fenbufen were the standards. The compounds containing 4-Cl, 4-NO2, 4-F and 4-MeO groups were equipotent to fenbufen, and the compound with a 3,4-di-MeO group was more potent than the fenbufen, and equal to sodium diclofenac [90] (Figure 8).

2-(1-adamantyl)-5-substituted-1,3,4-oxadiazole compounds 140 displayed strong dose dependent inhibition of carrageenan-induced paw edema with >50% inhibition at a concentration of 60 mg/kg. The compound with the 3,4-di-MeO group was more potent than the indomethacin standard [91], (Figure 8). Burbuliene and co-workers [92] investigated anti-inflammatory activity for 5-[(2-di-substituted diamino-6-methyl-pyrimidin-4-yl)sulphanylmethyl]-3H-1,3,4-oxadiazol-2-thione derivatives 141 and found that some were more potent than ibuprofen, (Figure 8). Other compounds with anti-inflammatory activity are also included: 142 [93], 143 [94], 144 [95], 145 [96], 146 [97], 147 [98], 148 [99] and 149 [100], (Figure 8).

3.4. Analgesic Activity

5-(2-(2,6-Dichlorophenylamino)benzyl)-N-(4-fluorophenyl)-2-amino-1,3,4-oxadiazole (150) was more potent in an evaluation of its analgesic activity than sodium diclofenac with a maximal analgesic activity of (81,86%) [27] (Figure 9). The compound 151 containing the 2,4-dichlorophenyl group, present at the second position of the oxadiazole ring, showed a maximal activity of (70.37 ± 1.67%), almost equivalent to that of the ibuprofen standard (73.52 ± 1.00%) [101] (Figure 9). Compounds 138, 139, 144, 146, 147, 148 (Figure 8) also display analgesic activity. Compound 139 with the R=4-F group showed a maximal analgesic activity of (72.52%), better than both sodium diclofenac (70.32%) and fenbufen (54.1%) [90].

Figure 9

1,3,4-Oxadiazoles with analgesic activity.

3.5. Antitumor Activity

Savariz and co-workers [102] synthesized and evaluated the in vitro antitumor activity of new Mannich bases. Among the compounds studied, compound 152 showed potent activity against melanoma (UACC-62), and lung (NCI-460) cell lines with GI50 values of 0.88 and 1.01 mmol/L, respectively, (Figure 10). Liu and co-workers [103] synthesized and reported the anti-proliferative and EGFR inhibition properties of a series of 2-(benzylthio)-5-aryloxadiazole derivatives. Compound 153 showed potent biological activity (IC50 = 1.09 μM for MCF-7, and IC50 = 1.51 μM for EGFR) (Figure 10).

Figure 10

1,3,4-Oxadiazoles with antitumor activity.

Ouyang and co-workers [104], and Tuma and co-workers [105] synthesized and evaluated various 1,3,4-oxadiazole derivatives as to their ability to inhibit tubulin polymerization and block the mitotic division of tumor cells. Compounds 154 and 155 exhibited potent activity. In vitro studies of compound 154 indicated that at nano-concentrations it interrupted mitotic division in breast carcinoma and squamous cell tumors, which included multi-drug resistant cells. In vivo studies of compound 155 displayed a desirable pharmacokinetic profile (with appropriate plasma levels after oral administration), and was significantly more effective than the taxane paclitaxel (Figure 10).

The anti-proliferative effects of 24 new 2,5-diaryl-2,3-dihydro-1,3,4-oxadiazoline compounds (Type I) 157, and (Type II) 158, analogous to combretastatin-A4 (156) were evaluated in murine L1210 leukemia cells (Figure 10), and also in murine B16 melanoma cells. Combretastatin-A4 is the most potent of natural combretastatins. Early studies have shown that it inhibits tubulin polymerization, and proliferation of murine, and human cancer cells. Type I compounds with R1=R2=R4=R5=H, and R3=Br groups, displayed an IC50 of 0.6 ± 0.7 μM. Type II compounds with R1=R5=H, and R2=R3=R4=OCH3 displayed an IC50 of 0.5 ± 0.06 μM. However, these compounds were substantially less potent than the compound 156 which displayed an IC50 of 0.003 μM (all for L1210 cells) [106]. Other compounds with antitumor activity are included: 159 [107], 160 [108] and 161 [109].

3.6. Antiviral Activity

On October 16, 2007, the US Food and Drug Administration (FDA) approved raltegravir (Isentress®, 162, Figure 11) for treatment of human immunodeficiency virus (HIV)-1 infection, in combination with other antiretroviral agents in treatment-experienced adult patients who have evidence of viral replication, and HIV-1 strains resistant to multiple antiretroviral agents. Raltegravir is the prototype of a new class of antiretroviral drugs known as integrase inhibitors [110].

Figure 11

Structures of raltegravir (162) and derivatives.

Seeking to identify more promising compounds than raltegravir, Wang and co-workers [111] synthesized a series of raltegravir derivatives by modifying the 5-hydroxyl group of the pyrimidine ring and evaluated them for anti-HIV activity. The 5-hydroxyl modification of raltegravir derivatives significantly increased their activity, which indicates the 5-hydroxyl group’s dispensability. Compound 163 with a sub-picomol IC50 value was the most potent anti-HIV agent among all of the derivatives synthesized, and thus emerged as a new and potent anti-HIV agent (Figure 11).

The inhibitory activity of the compounds 164 and 165 (Figure 12) against the human immunodeficiency virus type 1 (HIV-1) was determined using the XTT assay on MT-4 cells. Compound 165 was the most active among the compounds tested, producing 100, 43 and 37% reductions in viral replication at concentrations of 50, 10 and 2 µg/mL respectively. Compounds 164 with R=4-F, and 2-Br groups exhibited less anti-viral replication activity yet above 10% inhibition at concentrations of 2 µg/mL. All tested compounds were non-cytotoxic with CD50 > 100 µg/mL except compound 165 whose CD50 was 68 µg/mL [112].

Figure 12

1,3,4-Oxadiazoles with inhibitory activity against human immunodeficiency virus type 1 (HIV-1).

Iqbal and co-workers [113] reported inhibitory activity for compounds 166 and 167 (Figure 12) against the human immunodeficiency virus type 1 (HIV-1) which was also determined using the XTT assay on MT-4 cells. Compound 166 with the R=Cl group was the most active among the compounds tested, with 62, 21 and 14% reductions at concentrations of 50, 25 and 5 μg/mL, respectively.

Indinavir, another protease inhibitor is also used as a component of antiretroviral therapy for treating HIV infection and AIDS. Kim and co-workers [114] have synthesized and evaluated the protease inhibitory activity of a series of oxadiazoles 168 analogous to indinavir. All the compounds prepared inhibited protease activity at picomolar (IC50) concentrations (thus being more potent than the indinavir) (Figure 13).

Figure 13

1,3,4-Oxadiazoles with inhibitory activity against HIV and hepatitis C virus.

Johns and co-workers [115] reported antiviral activity (through inhibition of viral DNA integration) for new derivatives containing the 1,3,4-oxadiazole unit in combination with a ring system of 8-hydroxy-1,6-naphthyridine 169. Compound 170, containing a 5-methyl-1,3,4-oxadiazol-2-yl group at the C2 position of the quinoline ring, shows inhibitory activity against the hepatitis C virus NS3 protease [116] (Figure 13).

3.7. Antihypertensive Activity

Hypertension and cardiovascular disease are major causes of morbidity and mortality worldwide. Bankar and co-workers [117] reported the vasorelaxant effect of compound 171, 4-(3-acetyl-5-(pyridin-3-yl)-2,3-dihydro-1,3,4-oxadiazol-2-yl)phenyl acetate (Figure 14), in rat aortic rings by blocking L-type calcium channels. Bankar and co-workers [118] also investigated whether the correction of endothelial dysfunction is dependent on high blood pressure normalization; in deoxycorticosterone acetate (DOCA-salt), and NG-nitro-L-arginine (L-NNA) in hypertensive rats. Compound 172 is a T type Ca2+ channel inhibitor with an IC50 of 810 nM [119] (Figure 13).

Figure 14

Vasorelaxant activity of 1,3,4-oxadiazoles.

3.8. Enzyme Inhibitors

Leukotrienes (LTs) are potent inflammatory lipid mediators derived from arachidonic acid metabolism, and released from cells involved in inflammation. The synthesis of all LTs requires the action of the enzyme 5-lipoxygenase (5-LO). Inhibition of 5-LO reduces the production of both LTB4, and cysteinyl LTs (CysLTs); LTC4, LTD4 and LTE4. 5-LO inhibitors have therapeutic potential for the treatment of inflammatory processes. A new oxadiazole p-toluenesulfonate derivative 173 (Figure 15) containing an asymmetric carbon was identified as both a potent and selective inhibitor of 5-lipoxygenase (5-LO) by Ducharme and co-workers [120], and Gosselin and co-workers [121].

Figure 15

1,3,4-Oxadiazole as enzyme inhibitors.

Leung and co-workers [122] reported the discovery of a new class of disubstituted oxadiazoles 174 (Figure 15) from oleic acid derivatives with potent and selective inhibition of fatty acid amide hydrolase. Khan and co-workers [123] performed studies on inhibition effects on tyrosinase with 19 2,5-disubstituted-1,3,4-oxadiazole compounds, the compound 3-(5-(4-bromophenyl)-1,3,4-oxadiazol-2-yl) pyridine (175) with an IC50 of 2.18 µM was more potent than the standard L-mimosine (IC50 = 3.68 µM) (Figure 15).

Tomi and co-workers [124] reported a study with the bis-1,3,4-oxadiazole compound 176 that contains a glycine unit on the transferase activity of enzymes such as: GOT, GPT and γ-GT in serum. Compound 176 showed activation for GOT and GPT and inhibitory effects on the activity of γ-GT, (Figure 15).

Maccioni and co-workers [125] synthesized a set of 3-acetyl-2,5-diaryl-2,3-dihydro-1,3,4-oxadiazoles and tested them as inhibitors of human monoamine oxidase (MAO) A and B isoforms. None of the tested compounds displayed significant inhibitory ability for MAO-A. However, several compounds were identified as selective MAO-B inhibitors. Some of the tested compounds exhibit interesting biological properties with an IC50 for the B isoform ranging from micromolar to nanomolar values. Compounds 177 were active at inhibiting MAO-B at nanomolar concentrations (Figure 16).

Figure 16

1,3,4-Oxadiazoles showing inhibitors activity against enzymes.

The optimization study of the central heterocycle of α-ketoheterocycle inhibitors of fatty acid amide hydrolase realized by Garfunkle and co-workers [126], led to identification of the most potent inhibitors 178. 5-aminopyrimidinone R-keto-1,3,4-oxadiazole (ONO-6818) is representative of orally active nonpeptidic reversible inhibitors of Human Neutrophil Elastase (HNE), with potent Ki values in the nanomolar range (Figure 16) [127,128]. Selective human Granzyme B inhibitors 179 inhibit CTL mediated apoptosis [129], and compound 180 (EC50 = 3.7 nM) demonstrated balanced potency and PK profiles. The molecule also exhibited potent oral in vivo efficacy potentiating the cytotoxic agent temozolomide in a B16F10 murine melanoma model [130], (Figure 16).

Various other 1,3,4-oxadiazole derivatives show inhibitory activity against enzymes such as: 181 [131], 182 [132], 183 [133], 184 [134], 185 [135], 186 [136], 187 [137], 188 [138], 189 [139], 190 [140], 191 [141] and 192 [142], (Figure 17).

Figure 17

Other 1,3,4-oxadiazole derivatives as enzymes inhibitors.

A summary of other activities exhibited by compounds containing the 1,3,4-oxadiazole nucleus can be found in the Figure 18. For further biological and pharmacological properties of 1,3,4-oxadiazole, see references [143,144,145].

Figure 18

Other biological and pharmacological activities of 1,3,4-oxadiazoles derivatives.

4. Conclusions

This review, has summarized the synthetic methods and biological activities for 1,3,4-oxadiazole derivatives reported in the literature during the past twelve years. The main synthetic methods include: (1) cyclodehydration reactions of diacylhydrazines; (2) cyclization oxidative reactions of N-acylhydrazones; (3) the one-step synthesis from readily available carboxylic acids and acid hydrazides; (4) the reactions of hydrazides with orthoesters; (5) hydrazide reactions with carbon disulfide in basic medium; (6) reaction of tetrazoles with acid chloride or acid anhydride. Most research groups are still using these synthetic routes making use only of new reaction conditions such as: new cyclization reagents, new catalysts, polymeric supports and microwave radiation. Few innovative methods have emerged in recent years, highlighting the methods described by Ramazani and Rezaei [51] and Cui and co-workers [50]. Furthermore, the various synthetic methods exemplified may serve as a support for the planning of new molecules containing the 1,3,4-oxadiazole unit. The broad pharmacological profile of this class of compounds is evidenced by the numerous examples cited here. In each biological activity topic, we have only provided selected examples of molecules with relevant activity being that these molecules may serve as prototypes for the development of more active derivatives.